With a continuously growing population, it is becoming increasingly difficult to meet the world's energy needs as well as to control carbon emissions to kerb greenhouse gas emissions that are considered responsible for global warming phenomena. These concerns have triggered a drive towards a more efficient use of electricity in an attempt to reduce energy consumption.
One such area of concern is lighting applications, either in domestic or commercial settings. There is a clear trend towards the replacement of traditional energy-inefficient light bulbs such as incandescent or fluorescent light bulbs with more energy efficient replacements. Indeed, in many jurisdictions the production and retailing of incandescent light bulbs has been outlawed, thus forcing consumers to buy energy-efficient alternatives, e.g. when replacing incandescent light bulbs.
A particularly promising alternative is provided by solid state lighting (SSL) devices, which can produce a unit luminous output at a fraction of the energy cost of incandescent or fluorescent light bulbs. An example of such a SSL element is a light emitting diode.
A problem associated with SSL element-based lighting devices it is far from trivial to produce a lighting device having an appearance that is comparable with traditional lighting devices such as incandescent and fluorescent light bulbs. Because customers are used to the appearance of such traditional lighting devices, acceptance of SSL element-based lighting devices typically is largely dependent on the similarity of the appearance of the device in operation when compared to such traditional lighting devices. An appearance that is dissimilar to traditional lighting devices can hamper the market penetration of the SSL element-based lighting devices because customers may dislike the different appearance of such devices. This is for instance problematic in tubular lighting devices based on SSL elements such as tubular light bulbs.
An example of such a prior art tubular lighting device is shown in FIG. 1. The lighting device 10 comprises a tubular body 20 having an inner volume comprising a printed circuit board 30 onto which a plurality of LEDs 32 are mounted at regular intervals. The LEDs 32 act as a point light sources, which can give the lighting device 10 a spotted a luminous appearance, which is notably different to the appearance of a fluorescent tube, which typically produces a substantially homogeneous or uniform luminous output.
In order for the lighting device 10 to produce a more uniform luminous output, the tubular body 20 may act as a diffuser, for instance by forming the tubular body 20 from a homogeneously diffused plastic or by providing a glass or plastic tubular body 20 with a diffuser coating. Such a diffuser may furthermore be desirable to prevent the LEDs from being directly observable, e.g. to prevent glare. However, high levels of diffusion may be required in order to generate the desired uniform luminous distribution. This is for instance the case if the lighting device 10 comprises a relatively small number of LEDs 32, in which case the LEDs 32 are spaced apart by relatively large distances. If such high levels of diffusion are required, this means that the light generated by the LEDs 32 typically is reflected several times inside the tubular body 20 before exiting this body. This can significantly reduce the optical efficiency of the lighting device 10, which is undesirable.
Moreover, approximately only half of the circumference of the tubular body 20 acts as a light exit window due to the fact that the printed circuit board 30 prevents light generated by the LEDs 32 to be reflected towards the arcuate section of the tubular body 20 underneath the printed circuit board 30, i.e. the part of the tubular body 20 that is not directly exposed to the luminous output of the LEDs 32. FIG. 2 depicts a cross-sectional light distribution plot of the lighting device 10 of FIG. 1 and FIG. 3 depicts a light distribution plot of the lighting device 10 of FIG. 1 along the tubular body 20, from which it is clear that the luminous distribution produced by the lighting device 10 is limited to a range of viewing angles of approximately 180° due to the presence of the planar printed circuit board 30 extending across the width of the tubular body 20.
JP 2010-272496 (A) discloses a LED fluorescent illumination apparatus having a tubular body composed of a first arcuate section made of a translucent synthetic resin and a second arcuate section made of a metal. An inner wall having a horizontal plane and a pair of inclined planes is located inside the tubular body with the horizontal plane positioned closer to the second arcuate section than the tubular body centre. LED light-emitting devices and phosphor are mounted on the upper surface of the horizontal plane of the inner wall. Generated fluorescent light is radiated from almost all the surface of the inner wall to the first tubular body such that a diffused light output is produced over an increased angular distribution compared to the lighting device 10 in FIG. 1. However, this apparatus still requires heavy diffusion to obtain the desired luminous distribution, which reduces the efficiency of the apparatus.